Unexamined Japanese Patent Publication Nos. SHO 56-165515 and SHO 54-54957, Unexamined Japanese Utility Model Publications Nos. SHO 54-5798 and SHO 54-62538, etc. each disclose double-stem indirect extrusion presses of prior art which comprise a billet accommodating container, and a press stem and a die stem arranged on the center line of extrusion on opposite sides of the container and by which a billet (solid or hollow) placed into the container is upset by the two stems in pressing contact with the opposite ends of the billet and extruded into a rod or tubular product through a die on the die stem by subsequently moving both the press stem and the container together toward the die stem and causing the press stem to apply pressure on the billet. Now, with reference to FIG. 1 showing an indirect extruder of the invention, the structure and operation of the press will be described generally. Arranged on the center line of extrusion X--X in FIG. 1(I) are a press platen thereof, although these members are not shown. The two members are connected together by tie rods. The press platen is provided with a die stem 7 as illustrated which is fixed with respect to the direction of the extrusion center line X--X. Although unillustrated, the press frame on the other side is provided with a series of extrusion pressure applying means, such as main cylinder, main ram and crosshead, as already known. The crosshead is provided with a press stem 6 as shown which is movable forward or backward along the extrusion center line X--X for the application of pressure as illustrated. Interposed between the press stem 6 and the die stem 7 is a container 1 mounted on a container holder 2 which is movable forward or backward along the extrusion center line X--X by an unillustrated container hydraulic coupling cylinder or like shift cylinder. A solid or hollow billet 5 is supplied to the container 1, for example, by an illustrated movable billet loader 9 or the like. FIG. 1(I) shows no die for the die stem 7 because a die 8 can be removably attached thereto by a die handling robot 10 or the like as seen in FIG. 1(II). The die stem 7 may of course have the die 8 as attached to its front end in advance to achieve the same result. Disposed at the rear end of the container holder 2 is a shearing device comprising a shear blade 3 and a shear cylinder 4 as already known. With such an indirect extrusion press of the double stem type, the billet 5 is first positioned in alignment with the center line X--X by the billet loader 9, then placed into the container 1, for example, by moving the container 1, thereafter upset by the press stem 6 and the die stem 7 at the opposite ends of the billet 5, and extruded into a rod product or the like through the die 8 on the die stem 7 by subsequently moving the container 1 and the press stem 6 together toward the die stem 7. A tubular product is similarly obtained by using a hollow billet and a mandrel arranged in the die stem 7 concentrically therewith, or by using a solid billet and a piercing mandrel or the like, as already known. The extrusion press of this type, although generally used for producing rods and tubes, has the following problems.
With the indirect extrusion press of the prior-art double stem type, the billet 5 in the container 1 is upset by the pressure applied by the two stems 6 and 7 to the billet ends axially thereof, so that the yield phenomenon of the billet takes place first at the opposite billet ends. Consequently, the air present in the space between the inner surface of the container 1 and the other surface of the billet 5 (especially, the air remaining in the midportion of the space) fails to escape, thus blistering the surface of the extruded product owing to the remaining air and degrading the surface of the product. To eliminate this drawback, a taper heating method is used in which the billet 5 is locally heated to a higher temperature toward the die stem 7, but the method is not fully effective for elongated billets or for billets to be extruded at a small ratio, while the method required providing equipment for operation.
While the extrusion of a solid billet into a rod product has been described, similar problems are encountered in extruding a hollow billet into a tubular product or in extruding a solid billet into a tubular product by piercing.